Clock and data recovery (CDR) in digital signal processing (DSP) based optical receivers is challenging if the received signal is distorted by chromatic dispersion, polarization mode dispersion or non-linear effects. Traditional digital clock recovery schemes are too complex to be implemented at data rates typically used in optical communications.
Clock and data recovery in digital signal processing (DSP) based optical receivers can be performed either in analog domain, digital domain, or in a mixed-mode fashion.
In an analog clock and data recovery scheme, the unprocessed signal before digitization is used for extracting a clock signal. This imposes limits on the maximum signal distortion (chromatic dispersion, polarization-mode dispersion, non-linearity) a receiver can handle independently of post-compensation.
In a digital clock and data recovery scheme, the digitized signal is processed by (1) extracting the actual clock signal, and (2) re-processing the sampled data so that it reflects the timing estimate extracted in the clock recovery scheme. Clock signal extraction in the digital domain is typically based on FFT-based tone extraction, while data recovery is typically performed using an interpolation filter with adjustable coefficients or a using a bank of interpolation filters with fixed coefficients. The adjustment of the coefficients in the first case or the selection of the output filter in the second case is based on the phase of the recovered clock.
In a mixed-mode clock and data recovery scheme, a control signal is fed back to an oscillator that creates a clock signal for an analog-to digital conversion based on the digitally recovered clock.